Apparatus and method for investigating a sample

ABSTRACT

An apparatus for investigating a sample comprising a source of a beam of radiation, a detector for detecting a beam of radiation reflected by or transmitted through a sample to be imaged, an optical subsystem for manipulating the beam between the source and detector and means for translating the optical subsystem along a first translation axis relative to the source and detector to scan the beam across the sample, wherein the source and the detector are on opposite sides of the subsystem and the beam from the source and the beam reflected or transmitted each enter and exit the subsystem in a direction parallel to the first direction of translation. The apparatus is also suitable for maintaining the relative phase of two beams of radiation, during translation of an optical subsystem.

This invention relates to the field of investigating and imagingsamples. More specifically, the invention is concerned with suchapparatus which scan a beam of electromagnetic radiation relative to thesample. The present invention is primarily intended for use in theso-called terahertz regime, this is typically thought of as being therange of frequencies from approximately 25 GHz to 100 THz.

It is known to use terahertz radiation to obtain spectra and images ofsamples. In particular, EP-A-0 727 671 discloses the use of a pulsedbeam of terahertz radiation to investigate a sample. The pulsed beamcomprises a plurality of different frequency components.

Materials, in general, have a frequency dependent response to radiation.By analysing the frequency components of the terahertz radiation in thetime domain or frequency domain, an image of the sample can begenerated.

While the present invention is suitable for use to obtain spectra from anumber of spatially distinct points on a sample, it is primarilyconcerned with the production of images of a sample. An image of asample can be generated by scanning the sample relative to the focus ofa beam. In the case of a large sample, it is not practical to move thesample, thus the focus of the beam must be scanned instead.

This has been alluded to in EP-A-0 727 671 which discloses focusing asignal source on distant points and moving the sample or moving thesource and detector across the sample. However, no indication is givenas to how this may be achieved. Due to the manner of operation of suchan imaging system, it is not apparent how the radiation may be scannedover a sample, while still being able to obtain meaningful results.PCT/GB00/00632, by the present applicant, discloses a method andapparatus for imaging a sample using terahertz radiation. The presentinvention builds on this and offers increased accuracy for imaging.

When considering the problems with scanning the beam relative to thesample it is important to understand what happens when a sample isirradiated by radiation. Electromagnetic radiation is a wave phenomenaand hence it is characterised by both amplitude and phase. Thusinformation regarding the material of the sample, and its internalmake-up can be found by measuring the change in amplitude and phase ofthe radiation introduced by the sample at different frequencies. Thephase is related to the propagation path length or propagation time ofthe radiation.

When the sample is scanned through the beams, the beams and theirassociated optics do not move, thus it is possible to easily measure thephase change caused by the sample. However, once the beam itself isscanned, problems arise in how to accurately determine the phase shiftintroduced by the sample.

It is an aim of the present invention to alleviate, or partiallymitigate some of the problems associated with the prior art. Inparticular, it is an aim of the present invention to provide anapparatus which allows scanning of a beam of radiation over a sample inat least one dimension while taking account of any change in time delayintroduced by the system itself, so that the delay introduced by thesample may be found.

According to a first aspect of the invention, there is provided anapparatus for investigating a sample, comprising a source of a beam ofradiation, a detector for detecting a beam of radiation reflected by ortransmitted through the sample, an optical subsystem for manipulatingthe beam between the source and detector, and means for translating theoptical subsystem along a first translation axis relative to a fixedreference point to scan the beam across the sample, wherein the beamfrom the source enters the subsystem on one side of the subsystem in adirection parallel to the first translation axis, and the beam reflectedor transmitted exits the subsystem on the opposite side of the subsystemin a direction parallel to the first translation axis.

The subsystem comprises at least one element for manipulating the beamor pulse of radiation which may be translated along a translation axisrelative to sample. Generally, the subsystem will comprise a pluralityof elements which may be translated together in unison along the desiredtranslation axis. Preferably, the subsystem will comprise at least oneelement configured to direct radiation onto the sample and at least oneother element configured to direct radiation reflected from the sampleinto the detector.

The beam enters and exits the subsystem in the same direction andparallel to the axis of translation and the subsystem is moveable alongthe translation axis. Thus, the path length travelled between the sourceand detector is constant regardless of the scanning position of thebeam. Specifically, if the subsystem is moved away from the source, theextra distance travelled between source and subsystem is compensated bythe shortening of the distance travelled between subsystem and detector.

The above arrangement keeps the path length of the irradiating radiationconstant regardless of the scanning position of the beam. Thus,providing that the detector is provided with some information concerningthe phase of the radiation leaving the source, the phase differenceintroduced by the sample can be determined.

The detector may be given information about the phase of the radiationleaving the source by a number of different methods. For example, thesource and detector may both be provided with a synchronised clocksignal; Preferably, a reference beam is used which does not pass throughthe sample and which has a phase related to that of the beam ofirradiating radiation.

In apparatus according to the above aspect of the present invention, thepath length of the reference beam will be fixed as the path length ofthe irradiating radiation is fixed and a known phase between the twobeams can be maintained. However, it is also possible to design a systemwhere the path length of the irradiating radiation changes, but wherethe path length of the reference beam changes by a corresponding amountThis arrangement still allows the detector to measure the change inphase between the irradiating beam and reference beam.

Thus, in a second aspect, the present invention provides an apparatusfor investigating a sample, comprising a source of a beam of radiation,a source of a reference beam, an optical subsystem for manipulating thesource beam means for translating the optical subsystem along a firsttranslation axis relative to the sample to scan the beam across thesample and a detector for detecting the reflected or transmitted beamwherein source and reference beams each enter the subsystem in adirection parallel to the first translation axis.

As the subsystem moves, the relative phase of the source and referenceprobe as they enter the subsystem will stay the same so that the phaserelationship between the two beams is altered only by the source beamimpinging on the sample. In this aspect, the reference beam is notstationary relative to the detector, but rather to the source. Thisallows accounting for the phase change introduced by the apparatus as itmoves to be made automatically, and allows a comparison of the sourceand reference beams at the detector to detect the phase change caused bythe delay introduced by the sample. This can be done at manyfrequencies, the phase change at each frequency not necessarily beingthe same, due to the frequency dependent refractive index of the sample.

The sample is preferably placed independent of the subsystem so that thesample may remain stationary while the subsystem moves the manipulatingelements relative to the sample.

The above description has concentrated on systems which scan in just onedirection However, it is possible to put two such systems inside oneanother in order to scan in two or more directions.

Thus, preferably, a second optical subsystem is provided formanipulating the source beam between the source and the detector, thefirst subsystem being contained within the second subsystem. Preferably,there are means for translating the second optical subsystem relative tothe sample, to scan the source beam across the sample along a secondtranslation axis, wherein the source beam enters the second subsystem ina direction parallel to the direction of translation of the secondsubsystem.

Preferably, the first and second translation axes are not parallel, sothat a focus of the beam may be scanned in two dimensions and a twodimensional image of the sample may be constructed. However, a systemwhere there are two subsystems which scan in the same direction might beuseful. For example, the first subsystem may have a limited movementrange but have fine control whereas the second subsystem has a muchlonger movement range, but which cannot be so finely controlled.

The second optical subsystem may be in accordance with the second aspectof the present invention, where the second optical subsystem comprisesthe detector. The reference beam and source beam entering the secondsubsystem parallel to a second translation axis relative to the sample.

The detector may be outside the second subsystem in the manner of thefirst aspect of the present invention. If this is the case then thereference beam and reflected or transmitted beam exit the secondsubsystem parallel to the direction of translation of the secondsubsystem. The reference beam and the reflected or transmitted beam bothtravel the same distance to the detector regardless of the position ofthe subsystems. Therefore, the phase shift in the reflected ortransmitted beam, introduced by the sample, relative to the referencebeam is maintained until the beams reach the detector.

The first subsystem which is inside the second subsystem will generallybe of the type described with reference to the first aspect of thepresent invention where the detector lies outside the subsystem.

Preferably the first and second directions of translation areorthogonal. This gives the advantage that the first and secondsubsystems can move independently, movement along one translation axisnot causing any movement along the other translation axis.

However, the first and second translation axes need not be orthogonalfor the invention to operate. The beam irradiating the sample may beincident on the sample at any angle, although in general the beam isincident on the sample along a direction largely orthogonal to the firstand second translation axes.

It is possible to incorporate first and second subsystems as describedabove into a third subsystem which moves along a third translationalaxis. This can be done by arranging the source outside the thirdsubsystem and the source beam, the third subsystem parallel to the thirdtranslational axis. The third subsystem would manipulate the beams so asto enter the second subsystem parallel to the second translational axis.In this case, the source and reference beams would each travel the samedistance, the change in distance due to movement of the second and thirdsubsystems will be the same for each beam. The apparatus will notintroduce any phase difference between the two beams when each subsystemis moved along its translational axis. Alternatively, the source andreference source could be mounted within the third subsystem and movedtogether along the third translation axis.

The above description has generally referred to systems where the beamsof radiation (e.g. the irradiating beam of radiation or the referencebeam) pass through free space and are reflected and focused by freespace optics. However, the benefits of the first and second aspects ofthe present invention can also be realised by using flexible waveguidessuch as optical fibres.

Thus, in a third aspect, the present invention provides an apparatus forinvestigating a sample is provided, comprising a source of a beam ofradiation, a source of a reference beam, an optical subsystem formanipulating the beam between source and detector, and means fortranslating the subsystem relative to the sample, wherein the referencebeam enters the subsystem through an electromagnetic radiation guide,one end of the guide being fixed with respect to the source, the otherend of the guide being fixed relative to the subsystem.

This aspect allows for the subsystem to be translated in threedimensions whilst maintaining the path length of the reference beam asit is retained within a guide, one end of which is fixed in relation tothe radiation sources, the other end fixed relative to the subsystem.This ensures that the phase relationship between the source andreference beams entering the subsystem is maintained. Preferably, thesource is also outside the subsystem and the source beam enters thesubsystem through an electromagnetic radiation guide. Preferably, theelectromagnetic radiation guides are optical fibres. Any guide may beused which maintains the absolute path of the beams, and which maintainsthe coherence of the beams.

In apparatus according to any of the above three aspects of theinvention, the detector may be a direct detector of THz radiation or itmay be of the type which converts THz radiation into an easily readablesignal.

For example, the detector may comprise a non-linear crystal which isconfigured such that upon the radiation of a probe beam and a THz beam,the polarisation of the probe beam is rotated. The probe beam can be ofa frequency which can be easily measured (for example near infra-red).Typical crystals which exhibit this effect, the so-called “AC Pockels”effect are GaAs, GaSe, NH₄H₂PO₄, ADP, KH₂PO₄, KH₂ASO₄, Quartz, AlPO₄,ZnO, CdS, GaP, BaTiO₃, LiTaO₃, LiNbO₃, Te, Se, ZnTe, ZnSe, Ba₂NaNb₅O₁₅,AgAsS₃, proustite, CdSe, CdGeAs₂, AgGaSe₂, AgSbS₃, ZnS, organic crystalssuch as DAST (4-N-methylstilbazolium. This type of detection mechanismis generally referred to as ‘Electro-optic sampling’ or EOS.

Alternatively, the detector could be a so-called photoconductingdetector. Here, the detector comprises a photoconductive material suchas low temperature grown GaAs, Arsenic implanted GaAs or radiationdamaged Si on Sapphire. A pair of electrodes, for example in a bow-tieconfiguration or in a transmission line configuration are provided on asurface of the photoconductive material. When the photoconductivematerial is irradiated by the reflected radiation and also, the probebeam, a current is generated between the two electrodes. The magnitudeof this photovoltage current is an indication of the magnitude of theTHz signal.

The present invention is primarily intended for use in the THz regime.Although it is possible to generate terahertz radiation directly, themost efficient terahertz generation can generally be achieved byconverting an input beam or pump beam into a terahertz beam. Therefore,a frequency conversion member is preferably provided between the sourceof the pump beam and the sample. This frequency conversion device may besituated anywhere between the source and sample and can be used toirradiate the sample with radiation of a frequency different to that ofthe pump beam. It is therefore possible to irradiate the sample withradiation frequencies which are not easily directly produced, in asimplified manner.

There are many possible options for the frequency conversion member. Forexample, the frequency conversion member may comprise a non-linearmember, which is configured to emit a beam of emitted radiation inresponse to irradiation by a pump beam. Preferably, the pump beamcomprises at least two frequency components, (or two pump beams havingdifferent frequencies are used), the non-linear member can be configuredto emit an emitted beam having a frequency which is the difference ofthe at least two frequencies of the pump beam or beams. Typicalnon-linear members are: GaAs or Si based semiconductors. Morepreferably, a crystalline structure is used. The following are furtherexamples of possible materials:

NH₄H₂PO₄, ADP, KH₂PO₄, KH₂ASO₄, Quartz, AlPO₄, ZnO, CdS, GaP, BaTiO₃,LiTaO₃, LlNbO₃, Te, Se, ZnTe, ZnSe, Ba₂NaNbsO₁₅, AgAsS₃, proustite,CdSe, CdGeAs₂, AgGaSe₂, AgSbS₃, ZnS, GaSe or organic crystals such asDAST (4-N-methylstilbazolium).

In order to produce an emitted beam having a frequency in the THzregime, preferably the at least two frequencies of the pump beam orbeams are in the near infra-red regime. Typically, frequencies between0.1×10¹² Hz and 5×10¹⁴ Hz are used.

Alternatively the frequency conversion member is a photoconductingemitter, such an emitter comprises a photoconductive material such aslow temperature grown or arsenic implanted GaAs or radiation damaged Sior Sapphire.

Electrodes which may be of any shape such as a dipole arrangement, adouble dipole arrangement, a bow-tie arrangement or transmission linearrangement are provided on the surface of the photoconductive material.At least two electrodes are provided. Upon application of a bias betweenthe electrodes and irradiation of a pump beam(s) having at least twodifferent frequency components, a beam of radiation is emitted having afrequency different to that of the at least two frequency components ofthe pump beam or beams.

Preferably, the incident beam is pulsed beam comprising a plurality offrequencies. If a pulse having a plurality of frequencies passes into asample and onto a detector, the various frequencies will not all arriveat the detector at the same time due to the frequency dependent responseof the sample. Thus, a scanning delay line is inserted into the path ofeither the probe beam or pump beam. By plotting the signal against thedelay time (of the scanning delay line), the detected waveform can bemeasured.

In the above described apparatus, at least part of the apparatus, theoptical subsystem, moves relative to the sample. In order to achievethis, the apparatus preferably further comprises mounting means whichare used to mount the sample. In some situations, it will beadvantageous to provide a member which is substantially transparent tothe radiation of interest, in this THz radiation for the sample to besit upon. Thus, the radiation must pass through this member in order toirradiate the sample. This member can potentially give rise to internalreflections. Therefore, there is a need to try and at least partiallyremove reflections due to the member.

In a fourth aspect, the present invention provides a method ofinvestigating a sample, a method comprising:

-   (a) irradiating a sample with an irradiating beam of radiation,    through a member which is substantially transparent to the beam of    radiation;-   (b) detecting the radiation reflected from the sample;-   (c) irradiating the said member in the absence of the sample; and-   (d) detecting radiation reflected from the member during step (c);-   (e) subtracting the signal measured by the detector during step (d)    from the signal measured by the detector during step (b).

Steps (a) and (b) can be continually repeated during a scanningoperation and the same signal measured in step (d) can be usedregardless of the current scanning position.

Steps (c) and (e) generally referred to as measuring the “base line”signal and the above method is generally termed base line subtraction.The base line signal can be measured before or after scanning thesample. For example, if the system is used to measure a plurality ofsamples, then the base line signal only needs to be measured once.Generally, the above system will be used during pulsed imaging where thebeam of radiation comprises a plurality of frequencies.

The method of subtracting the signal due to the window from the samplecan be performed in the time or frequency domain.

In a fifth aspect, the present invention provides an apparatus, theapparatus comprising an emitter for emitting a beam of radiation toirradiate the sample; a detector for detecting radiation reflected fromthe sample; a member, which is substantially transparent to radiation,located between the sample and the emitter and detector; means forsubtracting a predetermined signal detected by the detector from afurther signal detected by the detector.

It may also be desirable to measure a reference signal and divide thesignal measured in step (b) by this reference signal. For example, thisreference signal could be obtained by replacing the sample with a knownreference, for example a silver mirror.

The baseline signal can be subtracted from both the sample signal andalso the reference signal. Alternatively, the baseline and referencesignals can change roles and the reference signal can be subtracted fromboth the sample signal and the baseline signal, the reference subtractedsample signal being divided by the reference subtracted baseline signal.Signals may be subtracted from one another in the time or the frequencydomain. Division of any signals should be performed in the frequencydomain

The obtained THz signal regardless of whether or not it has beensubjected to baseline subtraction or division by a reference signal ispreferably filtered.

Thus, in a sixth aspect, the present invention provides method ofinvestigating a sample, the method comprising the steps of:

-   -   (a) irradiating a sample with a beam of pulsed radiation        comprising a plurality of frequencies;    -   (b) measuring the beam reflected by or transmitted through the        sample and obtaining a signal representative of the measured        beam;    -   (c) multiplying the signal of step (b) in the frequency domain        by the complex Fourier transform of function F(t), wherein F(t)        is a non-zero function whose integral between time limits t₁ and        t₂ is zero, where t₁ and t₂ are chosen on the basis of the time        delay introduced by the sample.

To clarify, the sample will introduce a time delay into the path of thebeam. Considering the case of transmission, if the sample has the sameproperties as free space, then the radiation will pass through thesample without any delay. However, if the sample (as it almost certainlywill) introduces a time delay into the beam, then t₁ will be set to anegative value which has a magnitude is equal to or larger than theexpected value of this time delay. t₂ will generally, be set to thepositive value of t₁.

Similarly, during reflection, t₁ will be set to a negative value whichhas a magnitude is equal to or larger than the expected value of thistime delay introduced by the pulse being reflected from the deepestpoint of interest in the sample. t₂ will generally, be set to thepositive value of t₁.

Generally, the method of the above aspect of the present invention willbe performed using the above described apparatus where the radiation ismeasured using a reference beam and wherein a scanning delay line isintroduced into the path of the reference beam or irradiating beam inorder to measure the phase change introduced by the sample. In thissituation, t₁ and t₂ will be the negative and positive limits of thescanning delay line. These may be set to the duration of the pulse orpossibly a shorter time range.

Preferably function F(t) comprises a Gaussian component Generally, t₁and t₂ are symmetric about 0, preferably F(t) is also symmetric aboutzero.

As the signal is usually digitally sampled, strictly a summation isperformed as opposed to an integral.

A particularly preferable form of F(t) is provided by:

${F(t)} = {\frac{2}{\pi}\left\{ {\frac{{\mathbb{e}}^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{{\mathbb{e}}^{{- 2}{(\frac{t}{\beta})}^{2}}}{\beta}} \right\}}$where α and β are constants.

Preferable α is substantially equal to the shortest pulse length of thebeam of pulsed radiation and β is set to be much longer than the pulselength, typically 5 to 100 times the pulse length. However, both ofthese values will generally be optimised by the operator.

If β is greater than or comparable to the time which it takes theradiation to penetrate the sample to the point of interest then, F(t)can take the simplified form:

${F(t)} = {\frac{2}{\pi}\left\{ {\frac{{\mathbb{e}}^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{1}{T}} \right\}}$where α is a constant which is substantially equal to the shortest pulselength of the beam and T is substantially equal to the time which ittakes the beam of radiation to penetrate to the deepest point ofinterest in the sample.

Preferably data which has been baseline subtracted is filtered. Thus,preferably, step (b) comprises:

-   -   (b(i)) irradiating a sample with a beam of radiation, through a        member which is substantially transparent to the beam of        radiation;    -   (b(ii)) detecting the radiation reflected from the sample;    -   (b(iii)) irradiating the said member in the absence of the        sample; and    -   (b(iv)) detecting radiation reflected from the member during        step (b(iii));    -   (b(v)) subtracting the signal measured by the detector during        step (b(iv)) from the signal measured by the detector during        step (b(ii)).

The data has also been preferably divided by a reference signal beforefiltering. Thus, step (b) may comprise:

-   -   (b(i)) irradiating the sample with a beam of radiation, through        a member which is substantially transparent to the beam of        radiation;    -   (b(ii)) detecting radiation which is transmitted through or        reflected from the sample;    -   (b(iii)) irradiating the said member in the presence of a        reference sample having known reflection and/or transmission        characteristics; and    -   (b(iv)) detecting radiation which is reflected from or        transmitted through the reference sample during step (b(iii));    -   (b(v)) dividing the signal measured by the detector during step        (b(ii)) with the signal measured by the detector during step        (b(iv)).

More preferably the signal has been baseline subtracted and divided by areference signal prior to filtering. Thus step (b) may comprise:

-   -   (b(i)) irradiating the sample with a beam of radiation, through        a member which is substantially transparent to the beam of        radiation;    -   (b(ii)) detecting radiation which is transmitted through or        reflected from the sample;    -   (b(iii)) irradiating the said member in the presence of a        reference sample having known reflection and/or transmission        characteristics; and    -   (b(iv)) detecting radiation which is reflected from or        transmitted through the reference sample during step (b(iii));    -   (b(v)) irradiating the said member in the absence of the sample;    -   (b(vi)) detecting radiation reflected from the member during        step (b(v));    -   (b(vii)) subtracting the signal measured by the detector during        step (b(vi)) from the signal measured by the detector during        step (b(ii));    -   (b(viii)) subtracting the signal measured by the detector during        step (b(vi)) from the signal measured by the detector during        step (b(iv));    -   (b(ix)) dividing the signal obtained in step (b(vii) with the        signal obtained in step (b(viii)).

As explained above, the reference signal and the baseline signal areinterchangeable, thus, step (b) may comprise:

-   -   (b(i)) irradiating the sample with a beam of radiation, through        a member which is substantially transparent to the beam of        radiation;    -   (b(ii)) detecting radiation which is transmitted through or        reflected from the sample;    -   (b(iii)) irradiating the said member in the presence of a        reference sample having known reflection and/or transmission        characteristics; and    -   (b(iv)) detecting radiation which is reflected from or        transmitted through the reference sample during step (b(iii));    -   (b(v)) irradiating the said member in the absence of the sample;    -   (b(vi)) detecting radiation reflected from the member during        step (b(v));    -   (b(vii)) subtracting the signal measured by the detector during        step (b(iv)) from the signal measured by the detector during        step (b(ii));    -   (b(viii)) subtracting the signal measured by the detector during        step (b(iv)) from the signal measured by the detector during        step (b(vi));    -   (b(ix)) dividing the signal obtained in step(b(vii) with the        signal obtained in step (b(viii)).

Unwanted artefacts in the image may also be removed by appropriatechoice of the thickness of the emitter or detector such that artefactsdue to internal reflection within the emitter or detector occur atlonger delay times than those of interest in the sample.

Thus, in a seventh aspect, the present invention provides an apparatusfor investigating a sample with a beam of radiation, the apparatuscomprising an optically active element for irradiating the sample and/ordetecting radiation reflected from the sample, the optically activeelement having at least two interfaces, the distance between theinterfaces being great enough such that the beam of radiation takeslonger to travel between the interfaces than it does to travel from thesurface of the sample to the deepest point of interest within thesample.

The optically active element can be an emitter, detector or transceiver.

The apparatus may comprise two optically active elements, where thefirst optically active element is configured as an emitter and thesecond as a detector, wherein the second optically active elementcomprises at least two interfaces, the distance between the interfacesbeing great enough such that the beam of radiation takes longer totravel between the interfaces than it does to travel from the surface ofthe sample to the deepest point of interest within the sample.

The optically active element may be configured as an emitter which emitsthe beam of radiation having the desired frequencies in response toirradiation by at least one input beam having a different frequency.Such an emitter may comprise an optically non-linear material or aphoto-conductive material.

The optically active element may be configured as a detector whichdetects the beam of radiation having the desired frequencies using atleast one probe beam having a different frequency to that of the beam ofradiation. Such a detector may comprise an optically non-linear materialwhich is configured to alter the polarisation of the probe beam inresponse to irradiation with the beam of radiation or a photoconductivematerial which is configured to generate a current within thephotoconductive material in response to irradiation with both the beamof radiation and the probe beam.

The present invention will now be described with reference to followingnon-limiting embodiments, in which:

FIG. 1 is a schematic diagram of an imaging apparatus forming a firstembodiment of the present invention;

FIG. 2 is a schematic diagram of an imaging apparatus forming a secondembodiment of the present invention;

FIG. 3 is a schematic diagram showing an apparatus in accordance with athird embodiment of the present invention;

FIG. 4 is a schematic diagram showing an apparatus in accordance with afourth embodiment of the present invention;

FIG. 5 is a schematic diagram showing an apparatus in accordance with afifth embodiment of the present invention; and

FIG. 6 is a schematic diagram showing an apparatus in accordance with asixth embodiment of the present invention.

FIG. 7 shows an apparatus in accordance with a farther embodiment of thepresent invention;

FIG. 8 shows the apparatus of FIG. 7 and its control system;

FIG. 9 is a schematic of the principle optics used to image a samplethrough a window,

FIGS. 10 a, 10 b, 10 c, show THz radiation being reflected from awindow, a reference reflector and window a sample and windowrespectively;

FIG. 11 shows data from a sample measured in accordance with anembodiment of the present invention, the THz electric field (y-axis) isplotted against the time-delay of the signal (the x-axis), the raw datais shown by a dotted line, the solid line shows the same data afterbaseline subtraction.

FIG. 12 a shows a plot of electric field (y axis) in arbitrary unitsagainst time delay (x axis) arbitrary units for a THz scan of skin, thedata has been filtered and baseline subtraction has been performed, FIG.12 b shows the same data without baseline subtraction;

FIG. 13 is a schematic demonstrating the multiple internal reflectionswhich can occur within an emitter during THz reflection imaging;

FIG. 14 shows an image of a tooth where ghost images due to reflectionswithin the generation and detection crystals can be seen;

FIG. 15 shows the tooth of FIG. 14 measured using a thicker emitter;

FIGS. 16 a and 16 b show visible images of a human incisor, FIG. 16 ashows the outside image whereas 16 b shows the inside side image, frontand back faces are identified in FIG. 16 b;

FIGS. 17 a and 17 b show THz images of the from and back facesidentified in FIG. 16 b; and

FIG. 18 shows a THz image of a tooth indicating the enamel air andenamel dentine interfaces.

FIG. 1 shows schematically an apparatus according to the invention Theapparatus comprises a source 1, detector 2, a subsystem 3 containingfocusing optics 10, 11 and a substantially transparent mounting means 9for sample 7.

In this embodiment, the source 1 and detector 2 are provided on oppositesides of the subsystem 3. The source 1 provides abeam of irradiatingradiation 4 which is directed by the optics 10, 11 in optical subsystem3 onto sample 7. The irradiating radiation is then reflected 5 back intosubsystem 3 and the beam of reflected radiation 5 is then directed intodetector 2.

The subsystem 3 is translatable along a translation axis 6 and thesource beam 4 and reflected radiation beam 5 are parallel to thistranslation axis 6, as they respectively enter and exit subsystem 3.

The subsystem 3 includes a first parabolic mirror 10 which reflects andfocuses the source beam 4 to a focus 8 outside the subsystem 3. Ideally,the focus 8 is configured such that it is at the point of interest ofthe sample 7. The subsystem also comprises second parabolic mirror 11,which directs the beam of reflected radiation 5 to exit the subsystem 3parallel to the translation axis 6 and into detector 2.

Although parabolic mirrors are used as the optics which direct radiationto and from the sample 7, it is possible to use other optics which willfocus and/or reflect the source and reflected beams 4, 5 in the requiredmanner.

The sample 7 is placed on mounting means 9 which remain fixed withrespect to both the source 1 and the detector 2. The mounting means inthis particular example is a fixed window which allows the transmissionof the beam of irradiating radiation 4 and the reflected radiation 5.The sample is provided on the opposing side of the window 9 to theoptical subsystem 3.

As the subsystem 3 moves along the translation axis 6 it causes thefocus 8 in the beam 4, 5 to be scanned across the sample 7.

The above apparatus ensures that the path length travelled by the beam4, 5 remains constant, regardless of the position of the subsystem 3along the translation axis 6. Thus phase of the beam 4, 5 is notaffected by the movement of the subsystem 3 along the x translation axis6 in the absence of the sample.

In order to derive sensible information from the sample 7, there is aneed to measure the change in the phase of the source beam 4 caused bythe sample 7. Thus, detector 2 needs to know some information about thephase of the radiation leaving the source 1 The dotted line indicatesthat such information is required. A way of doing this is to use areference beam (not shown) which has a phase related to that of thesource beam 4 and which is received by the detector. Actually manydetectors of THz radiation require a reference beam in order to detectradiation received by the detector. Such detectors will be described inmore detail later.

In the apparatus of FIG. 1 the path length between the source and thedetector remains the same, thus a reference beam having a known pathlength can be provided to the detector 2. The use of a reference beamwill be described in great detail with reference to FIG. 3.

FIG. 2 shows, schematically, a second embodiment of the invention.However, in this case, two sources of radiation 101, 114 are provided,one for the source beam 104 and the other 114 for the reference or probebeam 104 for the detector 102. The detector 102 is provided within theoptical subsystem 103 and moves with the optical subsystem. Theapparatus is configured such that movement of the detector 102 causesthe same change in the path length of the source beam and the referencebeam, thus preserving the phase relationship between the two beams.

A first source 101 emits a source beam 104 and a detector 102 detectsthe beam of reflected radiation 105 after it has been reflected fromsample 107. A subsystem 103 is movable along translation axis 106. Thesource beam 104 is parallel to the translation axis 106 when it entersthe subsystem 103. The detector 102 is placed within the subsystem 103.A reference source 121 provides a reference beam 114 which enters thesubsystem 103 parallel to both the source beam 104 and translation axisof the subsystem 103 and is also detected by the detector 102.

The subsystem 103 includes a parabolic mirror 110 which reflects andfocuses the source beam 104 to a focus 108 outside the subsystem 103.The subsystem 103 also includes a further parabolic mirror 111, whichcollects reflected radiation from the sample 107 and directs it to thedetector 102. Other optical elements could be used instead of parabolicmirrors.

Sample 107 is placed on mounting means 109, which remains fixed withrespect to both the source 101 and the reference source 121. As thesubsystem 103 moves along the translation direction 106 the focus 108 ofthe source beam 104 is scanned across the sample 107.

The apparatus of FIG. 2 ensures that the phase difference between thesource beam 104 and the reference beam 114 remains constant as theyenter the subsystem regardless of the movement of the subsystem 103along the translation axis 106. This is because any change in the pathlength of the source beam 104 from the source to its entry to thesubsystem 103 due to the movement of the subsystem 103 along thetranslation axis 106 is the same as the change in the path length of thereference beam 114 from the reference source 121 to its entry to thesubsystem 103.

In both the above embodiments, the source 1, 101 is a source of coherentradiation, for example a laser, and more particularly a mode lockedTi:Sapphire laser may be used to produce near infrared radiation. Theapparatus of the present invention is primarily intended for use withTHz radiation. Although dedicated sources of THz radiation exist,generally, THz radiation is produce using a frequency conversion member,such as a non-linear crystal which is configured to generate THzradiation in response to irradiation by an input beam having a frequencydifferent to that of the THz radiation or a photoconductive materialwhich is configured to emit THz radiation upon application of a bias andirradiation by a input beam having a different frequency to that of theTHz radiation.

The source beam may be used directly or it may be passed through such afrequency conversion member. The part of the source beam which isincident on the frequency conversion member will be referred to as thepump beam and the part of the source beam emitted from the frequencyconversion member will be referred to as the THz beam. Although thisinvention is primarily intended for use in the THz regime, otherfrequencies could also be used.

The frequency conversion member may be driven by using a pump beamcomprising pulsed infra-red radiation which excites the member toproduce a single cycle terahertz pulse centred at round 1 THz, thespectrum being broad due to the short pulse length, while maintainingthe phase characteristics from the near infrared pulse. The frequencyconversion member may be placed in the path of the source beam eitherinside the optical subsystem or outside the optical subsystem 3, 103.

The parabolic mirror 10, 110 used to focus the source beam 4, 104 isused to a small focus 8, 108, typically less than from 1 mm whileensuring that radiation from across the whole cross section of the beam4, 104 arrives at the focus 8, 108 substantially simultaneously. Asimple off axis parabolic mirror may be used for near infrared radiationor terahertz radiation. No specialised optical equipment is required forthe manipulation of the terahertz pulses i.e. optical equipment which issuitable for infrared radiation manipulation may be used

The detector 2, 102 may be any detector suitable for detecting thefrequency of radiation supplied by the source 1, 101 and any referencesource 121. For example, in the case where the source 1, 101 is aterahertz frequency beam having a range of frequencies centred around 1THz, and a reference source 121 is supplied and is a near infrared beam,for example at a frequency of 1×10¹⁴ Hz, the detector may be either anon-linear crystal or a photo-conducting device. These are described ingreater detail in the embodiments relating to FIGS. 3 and 5 below.

In the embodiment of FIG. 2, the reference source 121 is a source ofcoherent radiation, but is not necessarily of the same frequency as thesource 101. A common source can be used for the source beam 104 andreference beam so that they are in phase.

A beam splitter (discussed in further embodiments) such as asemi-transparent mirror, or a prism, is used to split the beam toproduce the source beam 104 and reference beam 114.

The mounting means may be made out of any material substantiallytransparent to the radiation being used at the focus 8, 108. In the caseof a terahertz frequency apparatus, the mounting means is a crystalquartz window. It is possible that the mounting means 9, 109 has a holethrough it such that radiation can reach the sample 7, 107 from theoptical subsystem 3, 103.

The movement of the subsystem 3, 103 may be by any conventional means(not shown). For example, a stepper motor and/or rack & pinion system(not shown) may be employed. This allows accuracy of displacement andenables the position of the subsystem 3, 103 to be known by acontrolling system (not shown). Thus the position of the focus 8, 108can be determined with a high degree of accuracy. Typically, an accuracyof position of 0.01 mm is sufficient for the subsystem 3, 103, whenterahertz radiation is used to irradiate the sample 7, 107.

A third embodiment of the present invention will now be described withreference to FIG. 3. The apparatus of FIG. 3 allows a sample to bescanned in two orthogonal directions. For the purposes of this example,the two directions will be the x direction and y direction. Theapparatus uses a x optical subsystem (similar to that described withreference to FIG. 1) placed within a y optical subsystem (similar tothat described with reference to FIG. 2).

Source 221 comprises a mode locked laser generating sub 200 fs pulses inthe near infrared spectral range (approx. 1×10¹⁴ Hz). Specifically, aCr:LiSAF laser (100 MHz, 100 fs and 100 mW) is used. Other sources canbe used, for example, a mode locked Ti:Sapphire laser. These pulses ofnear infrared radiation are split using a beam splitter 220, into a pumppulse 224, and a reference pulse or probe pulse, 214.

The pump pulse 224 travels through scanning optical delay line 240,which introduces an oscillating delay into the pump pulse 224. Thescanning delay line comprises a retro reflection mirror 242 whichreflects the pump beam back on itself and onto planar mirror 243. Mirror243 is configured to reflect the beam out of the scanning delay line 240at right angles to itself and into the y optical subsystem 213 parallelto the translation axis. By moving the retroreflector 242 back and forthparallel to the direction on the pump beam, the optical path length isoscillated.

The irradiating radiation 224 then enters the y subsystem 213 parallelto the translation direction 216 of the y subsystem 213. On entering they subsystem 213, the pump beam 224 is reflected through 90° by mirror236 a and then through a further 90° by mirror 236 b such that the pathof the pump beam 224 is parallel to the direction which it enteredsubsystem 213. Mirrors 236 a and 236 b serve to direct the pump beam 224onto frequency conversion member 201 which converts the near infraredpump beam 224 into a beam of THz radiation 204.

Frequency conversion member 201 may comprise a non-linear crystal. If anon-linear crystal is irradiated by two different frequencies ω₁ and ω₂,radiation having a frequency which is the difference or the sum of thesefrequencies is outputted. Preferably, the pump beam and crystal ischosen such that the crystal outputs radiation having a frequency whichis the difference of the two frequencies impinging on the crystal.

As the frequency conversion member 201 is irradiated with infraredradiation ω₁ and ω₂, the electrons vibrate to emit radiation with a THzfrequency, the THz radiation ω_(THz)=ω₁−ω₂.

Often, such a frequency conversion member 201 will have phase matchingmeans in order to keep the transmitted THz signal and the incidentradiation in phase as they pass through the frequency conversion member201. (For example, see GB 2 343 964. Such phase matching can be achievedby providing the frequency conversion member 201 with a variation in itsrefractive index configured to keep the two signals in phase (at allpoints) as they pass through the frequency conversion member 201.

The frequency conversion member 201 may also be a so-calledphotoconducting emitter comprising a photoconducting material such aslow temperature grown GaAs or radiation damaged Si on Sapphire. A pairof electrodes are formed on a surface of the photoconducting material.THz radiation will be emitted having a frequency (or frequencies) whichis the difference in the frequencies of the photoconducting materialwith the pump beam and upon application of a bias between the twoelectrodes. (For further details on photoconductive emitters see U.S.Pat. No. 5,729,017).

The terahertz beam pulse 204, produced by the frequency conversionmember 201, is collimated using one or more parabolic mirrors 230, whichare suitable for use with terahertz frequency radiation. Parabolicmirror 230 serves to direct the radiation parallel to the x translationaxis 206 and to maintain the phase of the THz radiation across its crosssection. The incident THz pulse 204 then enters the x subsystem 203collimated and parallel to the x translation axis 206.

The x subsystem 203 performs in a similar manner to that described inthe first embodiment and comprises a set of elements for manipulatingthe pulse of radiation 204, 205 between its to entry and exit from the xsubsystem 203. The x subsystem can move along the x-translation axis.

The x subsystem 203 comprises a first parabolic minor 210, which focusesthe THz beam 204 to a focus (not shown) outside the x and y subsystems203, 213.

The sample (not shown) is placed on mounting means (not shown) whichremain fixed along the x translation axis 206 with respect to both thefrequency conversion device 201 and the detector 202, and fixed in the ytranslation axis 216 with respect to the source 221 such that the beamcan be scanned along the x and y axes of the sample.

The pulse 204 is incident on the sample (not shown) in a directionlargely orthogonal to the x and y translation axes 206, 216. However, itis possible to arrange the pulse 204 to be incident on the sample 207 atany angle. Radiation reflected by the sample is collected by y parabolicmirror 211 and is collimated parallel to the x translation axis 206 ofthe subsystem 203. The reflected THz radiation 205 exits the x subsystem203 in the same direction as the incident pulse 204 and on the oppositeside of the x subsystem 203.

The reflected THz radiation 205 which exits the x subsystem 203 back tothe y subsystem where it is collected by parabolic mirror 231 anddirected onto detector 202. Probe beam 214 is also incident on detector202.

Probe beam 214 from beam splitter 220 is directed though fixed delayline 241 and into the y subsystem 213. The probe beam 214 and THz beam205 need to reach the detector 202 in phase with one another. A scanningdelay line 240 is placed in the path of the pump beam 224 in order toscan the phase of the pump beam in order to measure the phase changescaused by the sample. A fixed delay 241 is also included in thereference beam 214 in order to ensure that the probe and reflected THzbeams can be brought into phase at detector 202. In order to allowtranslation of the y subsystem 213 along a y translation axis 216, thepulses 214, 224 enter the y subsystem 213 parallel to this y translationaxis 216, and preferably are collimated.

The difference in path length between the probe 214 and pump 224 beamsremains the same regardless of the position of the y subsystem 213 alongthe y translation axis 216 so that the phase relationship between theprobe and pulse beams is maintained.

Once the probe beam 214 enters the y subsystem, it is reflected through90° by first planar mirror 235 a and it is then reflected though afurther 90° by second planar mirror 235 b such that the probe beam 214is still travelling parallel to the y translation axis. Mirror 235 bserves to direct the probe beam through a hole in parabolic mirror 231such that the probe beam can be combined with the radiation reflectedfrom the sample for detection.

As the parabolic mirror 230 is arranged so that the terahertz pulse 204enters the x subsystem 203 collimated and parallel to the x translationaxis 206, and parabolic mirror 211 causes the pulse to be collimated andparallel to the x translation axis as it leaves the x subsystem 203,this allows the x subsystem 203 to be moved along the x translation axiswithout varying the total path length travelled by the terahertzradiation 204, and reflected/transmitted radiation 205. Hence no phasechange in the pulse is introduced at the detector 202 by virtue of themovement of the x subsystem 203 along the x translation axis 206.

The y subsystem 213 is translatable along the y translation axis 216,carrying the x subsystem 203 with it. The x subsystem 203 is within they subsystem 213 so that the x subsystem 203 may still be translatedalong the x translation axis 206, while the y subsection 213 istranslated along the y translation axis 213.

The detector is of the photoconducting type, comprising aphotoconductive detection member which may be, for example, GaAs,radiation damaged Si on Sapphire etc. The THz radiation is incident on afirst surface of the photoconductor 202. A pair of electrodes (notshown) are located on the photoconductor 202 on the first surface.

The probe beam 214 illuminates the surface of the detector between theelectrodes. The reflected Terahertz radiation 205 which is collected bythe lens induces a photocurrent through the region between theelectrodes which is being illuminated by the probe beam 214. Thecurrent, which can be detected by the electrodes, is proportional to thestrength of the THz field. The current is generated when the THzradiation arrives at the detector 202 in phase with the probe beam.

The travel of the terahertz radiation pulse in the sample (not shown)will induce a delay in the pulse. This delay will depend upon how farinto the sample 207 the pulse travels before being reflected, orotherwise directed back to the parabolic mirror 211. It will also dependon the refractive index of the sample at each frequency within thepulse. The pulse 205 is therefore broken up into different temporalelements by travelling through the sample, these different elementsarriving at the detector at different times. These elements are comparedwith the reference pulse.

However, all frequencies of the reference pulse arrive at the same time.In order to overcome this, the scanning time delay is used to introducea time delay in either the pump pulse or the probe pulse, (in this case,the pump pulse), so that it can be compared to parts of the pump pulsewith different time delays. Therefore, it is possible to detect thedelay introduced due to the sample at different frequencies, and theextent of the delay will give an indication of how far into a sample thepulse travelled at that frequency.

Also, because of the different reflectivities and absorptioncharacteristics for different frequencies of terahertz radiation for agiven sample, an indication of the material contained within a sample atvarious depths can be obtained. Since reflection of radiation will occurat a boundary between two materials with different opticalcharacteristics, an indication of the depth or thickness of, forexample, a surface layer may be determined.

Additionally, when the y subsystem 213 is translated along the ytranslation axis 216, the change in path length of each of the pump 224and probe 214 pulses is the same. Although the probe beam 214 does nottravel the along the same optical path as the pump pulse for irradiatingthe sample, any changes to the sample path length are kept the same asthose to the probe path length by the arrangement so that a comparisonof the phase of the two is possible, the only phase change which will beintroduced between the two pulses will be due to the sample 207.

The y translation axis 216 is orthogonal to the x translation axis 206so that independent movement in two dimensions of an orthogonalco-ordinate system is possible. Although the terms ‘x’ axis and ‘y’ axishave been used, the subsystems could be configured to move along any twoaxis regardless of whether or not they are orthogonal or non-orthogonal.

Although the above description has discussed reflecting radiation fromthe sample, to image the sample. In such an arrangement, the sample (notshown) is placed off axis from the parabolic mirrors 210, 211. However,transmission measurements are also possible if the sample is placedbetween the parabolic mirrors 210, 211. Other focusing elements could beused in addition to or instead of parabolic mirrors.

FIG. 4, shows schematically a fourth embodiment which is similar to thethird embodiment. However, in this embodiment, the reflected radiationand the probe pulse are incident on opposing sides of the detector 202.

To avoid unnecessary repetition, like reference numerals will be used todenote like features. Probe pulse 214 enters the y subsystem parallel tothe y translation axis and is reflected using mirrors 235 a and 235 bwhich direct the probe beam 214 parallel to the y translation axis. Theprobe beam then impinges on planar mirror 237 a which rotates the probebeam through 90° and onto mirror 237 b. Mirror 237 b reflects the beamthrough a further 90° such that the beam is reflected back parallel toits original path and onto the front of detector 202.

The path of the pump beam and reflected THz beam 205 remains identicalto that previously described with reference to FIG. 3.

The reflected THz radiation 205 from the sample is incident on the backsurface of the detector 202 and is collected by lens 242, which may behemispherical or have another shape. The lens is provided on the backsurface of detector 202.

On the opposing side of the detector 202 a pair of electrodes (notshown) is located. The probe beam 214 is incident on this side of thedetector 202 to the side of the detector which receives the reflectedTHz beam 205.

Alternatively, they may be triangular and arranged in the shape of abow-tie to from a so-called bow-tie antenna. They may also beinterdigitated electrodes at the centre of a bow-tie or spiral antenna.A transmission line arrangement of electrodes such as that disclosed inFattinger et al Appl. Phys. Lett. 54 4901 (1989) may also be used.

FIG. 5 shows schematically a fifth embodiment of the invention. Thisembodiment functions in largely the same manner as the embodiment ofFIG. 3 However, in FIG. 5, the detector 202 is replaced by a non-linearcrystal which is configured to operate as an electro-optic sampling(EOS) mixing crystal. To avoid unnecessary repetition, like referencenumerals have been used to denote like features.

In this embodiment, the probe pulse 214 and the reflected terahertzpulse from the sample (not shown) are both incident on the detector 202on the same side in the same manner as described with reference to FIG.3. The EOS mixing crystal 202 is not the final detection means, butinstead the EOS mixing crystal encodes information from terahertz pulse205 onto the probe pulse 214 by way of altering the polarising of theprobe pulse 214 as it passes through the EOS mixing crystal 202. Inorder for this to happen, the probe pulse 214 must arrive at thedetector 202 in phase path the reflected THz radiation 205.

An EOS mixing crystal 202 utilises the physical phenomenon known as theAC Pockels effect. The transmitted THz radiation 205 from the sample(not shown) is detected by passing the probe beam 214 through thecrystal 202 with the reflected THz pulse 205. The reflected THz pulse205 modulates the birefringence of the detection crystal by the ACPockels effect.

Prior to entry into the crystal 202, the THz pulse 205 and the probepulse 214 are polarised. Where there is no THz pulse 205, the probepulse 214 passes unaffected through the EOS mixing crystal 202. When theprobe pulse 214 exits the mixing crystal, it is passed into apolarisation detection system 232.

The details of the polarisation detection system are not shown as theywell known, see for example GB 2 343 964.

Since the relative phase information is contained in the one beamexiting the EOS mixing crystal 202, it is not important to have anyparticular distance from the detector 202 to the polarisation detector232, as the reference 214 and the reflected terahertz pulse 205 pulsesare now encoded in the same polarised pulse 234. Thus, the polarisationsensitive detection mechanism can be placed outside the x and ysubsystems as shown in FIG. 6.

FIG. 6 shows schematically a sixth embodiment of the invention. Thisembodiment is largely the same as the embodiment shown in FIG. 5, exceptthat the polarisation detector 232 is placed outside the y subsystem 213rather than inside the y subsystem 213. Here, the polarisation detectionsystem is provided on the opposite side of the y subsystem 213 to the tothe source 221. The polarisation detector 232 is fixed with respect tothe source 221.

The polarised pulse 234 is manipulated using mirrors 238 so that it isparallel to the y translation axis 216 as it exits the y subsystem 213.This means that the path length of the polarised pulse 234 does notchange as the y subsystem 213 moves along the y translation axis 216.

While the apparatus shown in FIGS. 3 to 6 have given examples of asystem which can scan the beam in two dimensions, it is readily apparentthat these techniques could be extended to three dimensions using thesame principles as given above.

FIGS. 7 and 8 show schematically a seventh embodiment of the invention.This embodiment includes a subsystem 53, within which an emitter 51 anddetector 52 are arranged.

The emitter 51 directs the radiation 54 towards a first parabolic mirror60 which reflects the radiation to sample 57 under investigation. Theradiation 55 reflected from or transmitted through the sample 57 isreflected by a second parabolic mirror 61 to the detector 52. The firstand second parabolic mirrors 60, 61 are both within the subsystem 53(such that they move with subsystem 53) and are fixed relative to theemitter 51 and detector 52.

The emitter 51 comprises a frequency conversion member which emitsradiation having the desired frequency, in this example THz radiation,in response to irradiation by an pump beam having a different frequency.In this example, the pump beam is supplied to the emitter using fibreoptic cable 62.

The detector operates using a probe beam. The probe beam has a knownphase relationship to that of the pump beam. The reference beam issupplied to the detector using fibre optic cable 63.

As the subsystem 53 moves to scan along any axis, the emitter 51 and thedetector 52 move with the subsystem 53. The path length of radiationtravelling from the emitter 51 to the detector 52 does not change.Neither does the path length of the probe pulse or pump pulse as theseare supplied via flexible fibre optic cables 62, 63 which can move withthe subsystem 53.

FIG. 8 shows the embodiment of FIG. 7 in more detail. Here, the samesource 71 is used as in the third and subsequent embodiments, i.e. aCr:LiSAF laser. As described in the third and subsequent embodiments,the source 71 produces a pulse which is split into a pump beam 65 andprobe beam 64 by beam splitter 72.

The emitter may be a direct emitter of THz radiation which does notrequire a pump pulse. However in this example, the reference pulse needsto still carry information about the phase of the radiation emitter bythe emitter to the detector.

Scanning delay line 73 is provided to cause an oscillating delay in theprobe pulse 64. The scanning delay line 73 is connected tomicroprocessor 80 such that the microprocessor knows the position of thedelay line at a point in time in order to analyse the detectedradiation.

The pump pulse optic fibre 62 extends between the beam splitter 72 andthe frequency conversion device 51. The probe pulse fibre 63 extendsbetween a delay line 73 (from the beam splitter 72) and the detector 52.The each optic fibre 62, 63 is fixed at one end relative to the beamsplitter 72, and at the other end relative to the subsystem 53.

The emitter 51 is a photoconductive emitter which emits radiation of thedesired frequency in response to irradiation with the pump beam and uponapplication of a bias across the electrodes. The bias voltage is appliedby antenna bias control voltage system 81. The control system 81 isconnected to the THz emitter via electrical cable 83.

The THz detector 52 generates a current in response to irradiation byTHz radiation in the presence of the probe beam. The measured current iscarried via cable 85 to signal detection and digitisation apparatus 87.The signal is then sent to microprocessor 80.

The subsystem 53 can be moved in-any direction relative to the sample 57because the path length of both source 65 and reference 64 beams arekept constant. This occurs because the optic fibres 62, 63 are of fixedlength and are fixed at each end. The relative phase between the source65 and reference 64 beams is therefore maintained within the system, sothat any path difference introduced is caused by the sample 57. Thisgives the advantage that more complicated optics can be used within thesubsystem 53 than parabolic mirrors 60, 61, as they do not have tocollimate radiation entering and leaving the subsystem 53, and can bespecific to the configuration within the subsystem 53. The subsystem 53is movable relative to the laser source 71, and the subsystem 53 may bescanned in three dimensions with no extra subsystems.

The data received by the detector can be analysed to find the depth towhich the sample pulse travels in to the sample. From this, a partialsample cross-section can be made up by scanning the sample pulse focalpoint along the translation axis. The delay time of the optical line canbe plotted against scanning position.

The embodiments described above may be used in a wide range of imagingapplications. As terahertz radiation has no shown detrimental effect onanimal or human tissue, it is possible to use it in a wide variety ofimaging techniques on various body and other parts, both after removaland in situ. For example, it is possible to obtain results for thethickness of the enamel on a tooth, whether from a human, or otheranimal. It is also possible to image layers of the skin and othertissues, including the stratum corneum and epidermis.

Many other imaging techniques are possible where the sample is at leastpartially transparent to terahertz radiation, and surface mapping ispossible where the sample is substantially opaque to terahertzradiation.

In FIG. 9, sample 1001 which is to be imaged is provided adjacent and incontact with window 1003. Window 1003 is positioned such that imagingradiation can only reach sample through window 1003 and radiationreflected from sample 1001 also has to pass through window 1003. Thesample imaged using radiation generated by THz emitter 1005 and theradiation reflected from the sample 1001 is detected using THz detector1007.

Radiation emitted from emitter 1005 is directed to the sample usingparabolic mirror assembly 1009. Parabolic mirror assembly 1009 alsodirects radiation reflected from sample 1001 into THz detector 1007.

THz beams 1011 a and 1011 b are emitted from THz emitter 1005. Both ofthese beams 1011 a and 1011 b are directed through window 1003 ontosample 1001 and reflected back onto parabolic mirror assembly 1009 andinto THz detector 1007. However, some of the beams which exit mirrorassembly 1009 will not pass through window 1003. Instead, some will bereflected off the underside 1013 of window 1003. The beams 1015reflected off the underside of window 1013 will also be detected by THzdetector 1007. As these beams have not been reflected from sample 1001,these beams contain no information about the sample and should beeliminated.

This problem of extraneous reflections are encountered in many systemsas windows are generally desirable. For example, a window may beprovided on a sealed case which encloses the imaging system such thatatmospheric gases/vapours are excluded from the system. A window canalso be used to define the plane of a “soft” sample (e.g. living tissue)or, to protect the THz system from external contamination.

FIG. 10 illustrates some steps which can be used in order to removereflections due to the window from the eventual image, so calledbaseline subtraction.

In FIG. 10 a, an image from the THz window 1003 is measured in theabsence of sample 1001. Initially, baseline subtraction will bedescribed using just the arrangements of FIGS. 10 a and 10 c. Then, theuse of a reference beam as shown in FIG. 10 b will also be explained.

In this specific example, the time domain waveform of the THz reflectedfrom just window 1003 as shown in FIG. 10 a is measured. The totalreflected THz radiation (reflected from both the sample 1001 and thewindow 1003) is measured as shown in FIG. 10 c.

The baseline signal, that measured in FIG. 10 a, is denoted by B(t), inthe time domain (where t represents time). A complex Fourier transformis then applied to the signal B(t), to obtain the baseline spectrumB′(ν) where (v) represents frequency. The time domain waveform measuredwith the sample of interest measured in FIG. 10 c is denoted by S(t).The complex for Fourier transform of this signal is then taken to obtainthe sample spectrum S′(ν).

The baseline subtracted waveform in the frequency domain is obtainedusing the following equation:S′(ν)−B′(ν)

This is then complex Fourier transformed in order to obtain the baselinesubtracted waveform or in the time domain.

Alternatively, baseline subtraction can be performed in the time domain.

FIG. 11 shows a plot of measured THz electric field in arbitrary unitsagainst the delay time in arbitrary units with baseline subtraction(shown as a solid line) and without baseline subtraction (shown as adotted line). The sample image here was a human finger tip. It wasimaged using THz radiation having a frequency range of 20 GHz–2.5 THz

In general, the image will be further defined by using a referencesignal and optionally, a filter function. FIG. 10 b shows the use of areference reflector 1014. The reference reflector 1014 is provided onthe opposing side of the window 1003 to the emitter and detector, in thesame position as the sample 1001. The reference reflector 1014 is usedfor spectral comparison. Typically, the reference beam will be obtainedusing a mirror or other sample with known and constant reflectioncharacteristics.

The time-domain waveform measured using the reference sample isgenerally represented by R(t). A complex Fourier transform is thenapplied to this time-domain waveform to obtain the reference spectrumR′(ν).

The baseline waveform in the frequency domain is subtracted from boththe sample waveform on the reference waveform. The baseline subtractedsample waveform is then divided by the baseline subtracted referencewaveform in the frequency in accordance with the following equation:

$\frac{{S^{\prime}(v)} - {B^{\prime}(v)}}{{R^{\prime}(v)} - {B^{\prime}(v)}}$

The above result is then complex Fourier transformed in order to obtainthe final time-domain waveform.

The same reference and baseline waveform can be used as the beam isscanned across the sample as the contribution from the window should notchange.

Also, the above procedure could be used where it is necessary tosubtract all reflections from other optical surfaces which are presentin the sub-system.

The reference and baseline measurements can also be reversed such thatthe reference waveform is measured from just the window and the baselineis measured with the reference reflector in place.

In general, a filter function will also be used F(t) and is complexFourier transform will be represented by a F′(ν).

A typically preferred filter function is used because the THz pulsesystem can generate and detect pulses comprising frequencies over somefinite range, typically from less than 100 GHz to over 3 THz. There is ahigh frequency limit above which the THz signal falls below the noiselevel of the detection system.

Similarly, the THz signal level falls below the noise level at lowfrequencies. Thus, there is a need to remove the high and low frequencynoise. A particularly preferable function for achieving this is:

${F(t)} = {{\frac{2}{\pi}\left\{ {\frac{{\mathbb{e}}^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{{\mathbb{e}}^{{- 2}{(\frac{t}{\beta})}^{2}}}{\beta}} \right\}}}$

The parameters α and β are selected to control the high and lowfrequency roll-off of the function. α is set approximately the shortestTHz pulse length (half cycle) obtainable within the THz system. β is setto be much longer than the THz pulse. In operation, the two parametersare optimised manually by the operator to obtain the best compromisebetween bandwidth and noise.

As the above function comprises two Gaussian functions with similarareas but opposite signs, the above function ensures that the integralof the filter function for all time is zero.

If the value of β is comparable to or greater than the total time-delayscan range, then an alternative function can be used:

${F(t)} = {{\frac{2}{\pi}\left\{ {\frac{{\mathbb{e}}^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{1}{T}} \right\}}}$where T represents the total range of delay times used i.e.T=T_(max)−T_(min). This ensures that the overall integral from T_(min)to T_(max) is always zero.

FIG. 12 a shows a reflection from skin where the THz electric field isplotted against the time delay in arbitrary units. The data shown inFIG. 12 a has been both baseline subtracted, and filtered. The referencebeam has also been used.

FIG. 12 b shows the same data as FIG. 12 a without the baselinesubtraction but with the filter function and reference.

It is also possible to remove or at least distinguish artefacts due tounwanted reflections by careful choice of the emitter and/or detectorthicknesses. In FIG. 13, sample 1001 has a first interface 1017 a firstinternal interface 1019 and a second internal interface 1021. It isrequired to find out the position of these three interfaces. However,there is no need to probe the sample deeper than interface 1021. The THzwaveform is generated within emitter 1005. In this simplified example,the emitter comprises a single crystal which has a first interface 1023and a second interface 1025 within the path of the beam. The first andsecond interfaces 1023 and 1025 are both capable of reflecting the THzradiation. In the ideal case, the THz radiation is generated within theemitter 1005, it follows path 1027 and is reflected off the furthestinterface 1021 (amongst other interfaces on sample 1001). The reflectedTHz signal is then directed into THz detector 1007 for detection. TheTHz detector 1007 may be a so-called photo-conducting detector where THzradiation 1027 in combination with a probe beam 1029 cause a current tobe generated in emitter 1007. Alternatively, it could be a so-called EOSdetector where the THz beam 1027 serves to rotate the polarisation ofthe probe beam 1029. By measuring the change in rotation of thepolarisation probe beam 1029, it is possible to measure the THzradiation.

The above is a rather over-simplified example where reflections onlyoccur from the interfaces 1017, 1019 and 1021 in the sample. However,typically, reflections will also occur within the emitter 1005 anddetector 1007. In FIG. 13, some of the THz radiation impinging on thesecond interface 1025 of emitter 1005 is transmitted through theinterface towards the sample 1001. However, some of this radiation isreflected back from interface 1025 towards first interface 1023 and thenis doubly reflected from interface 1023 through interface 1025 to thesample.

It is desirable to remove the secondary reflection. In this example, thetime which the THz radiation takes to be reflected from the firstinterface 1023 to the second interface 1025 following the path of theTHz beam, is longer than the time which it takes the THz radiation toenter sample 1001 through interface 1017 and to reflect from interface1021.

Thus, the generation crystal is thick enough so that THz pulsesreflected off the back of the crystal is measured at optical delay linesgreater than that of the sample structure of interest. In FIG. 13, theTHz beam radiation travels a distance D through sample 1001 in order toreach interface 1021. On entering the sample, the beam 1007 firsttravels through the first region which has a refractive index N₁ for adistance D₁ and then travels through the second region having arefractive index of N₂ for a distance d₂. The distance which the THzbeam travels from the first interface 1023 to the second interface 1025of the emitter is t. The refractive index of the emitter is n_(e). Forclear observation of the reflection from interface 1021, the followingequation must used:

$t = \frac{{N_{1}D_{1}} + {N_{2}D_{2}}}{n_{e}}$

Similarly, the pulse width t_(d) through the detector having arefractive index of n_(d) can be calculated in the same manner. Theemitter and/or detector may have multiple interfaces. In order to beable to clearly identify the signal due to reflections from each ofthese multiple interfaces, it is desirable for each of these multipleinterfaces to be positioned such that the THz radiation takes longer totravel between two adjacent interfaces than it does to travel throughthe sample to the interface of interest 1021.

FIG. 15 shows an image of a human tooth which has been measured to adepth of about 1 mm. The generation and detection crystals need to be atleast this thickness (in this case, the detection crystal is an ZnTe EOScrystal having a refractive index of 2.6 which is similar to that of therefractive index of dental enamel). The GaAs emitter has a refractiveindex of 3.6 which is slightly more than that of tooth enamel. Thus, aslightly thinner crystal can be used.

To obtain the shown image, a 0.5 mm emitter crystal was used and a 1 mmdetection crystal. In the shown image, the delay time is shown againstscanning position along a line along the tooth surface. The amplitude ofthe signal measured for the delay time is shown on a grey scale wherewhite indicates a strong positive THz field. The signal from the topsurface of the sample is shown for low delay times. A ghost image due toreflection within the emitter is shown at a delay time of about 7. Also,a further ghost image which probably arises due to multiple internalreflections within the sample 1001 is also shown.

FIG. 15 shows the same sample as FIG. 14 measured with an emittercrystal having a thickness of 1 mm. The signal due to the internal totalreflection has disappeared (while the true structural feature of thesample remains).

FIGS. 16 a and 16 b show visible images of a human tooth. The incisorhas been cut in two, FIG. 16 a shows an image of the outside of thetooth, whereas FIG. 16 b shows an image of the inside of the tooth, aback-side (top rectangle) and a front side (lower rectangle) areidentified on FIG. 16 b.

FIGS. 17 a and 17 b show THz images of the back and front sides of theincisor identified in FIG. 16 b.

FIG. 18 shows the THz image of the front side of the tooth of FIG. 16 b.A reflection artefact due to a 1 mm thick emitter crystal can be seen atthe top of the figure. The air/enamel and dentine/air interfaces canalso be seen

While above embodiments have made use of pulsed radiation, it will bereadily appreciated by one skilled in the art that continuous beams ofradiation could also be used, in part or in whole to replace the pulses,a pulse being a beam of short duration comprising multiple frequencies.

The present invention has been described above purely by way of example,and modifications can be made within the spirit of the invention. Theinvention also consists in any individual features described or implicitherein or shown or implicit in the drawings or any combination of anysuch features or any generalisation of any such features or combination.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise”, “comprising”, and thelike, are to be construed in an inclusive as opposed to an exclusive orexhaustive sense; that is to say, in the sense of “including, but notlimited to”.

1. An apparatus for investigating a sample, comprising: a source of abeam of radiation; a detector for detecting a beam of radiationreflected by or transmitted through the sample; an optical subsystem formanipulating the beam between the source and detector; and means fortranslating the optical subsystem along a first translation axisrelative to the sample to scan the beam across the sample; and means toprovide the detector with information concerning the phase of radiationleaving said source, wherein the beam from the source enters thesubsystem on one side of the subsystem in a direction parallel to thefirst translation axis, and the beam reflected or transmitted exits thesubsystem on the opposite side of the subsystem in a direction parallelto the first translation axis.
 2. The apparatus of claim 1, furthercomprising a frequency conversion device between the source and thesample.
 3. The apparatus of claim 1, wherein the detector comprises anon-linear optical crystal.
 4. The apparatus of claim 1, wherein thedetector comprises a photoconductor.
 5. The apparatus of claim 1,wherein the sample is irradiated with terahertz frequency radiation. 6.The apparatus of claim 1 wherein the source radiation has a frequency ofbetween 0.1×10¹² Hz and 5×10¹⁴ Hz.
 7. The apparatus of claim 1, whereinthe incident beam is pulsed.
 8. An apparatus for investigating a sample,comprising: a source of a beam of radiation; a detector for detecting abeam of radiation reflected by or transmitted through the sample; anoptical subsystem for manipulating the beam between the source anddetector; and means for translating the optical subsystem along a firsttranslation axis relative to the sample to scan the beam across thesample; wherein the beam from the source enters the subsystem on oneside of the subsystem in a direction parallel to the first translationaxis, and the beam reflected or transmitted exits the subsystem on theopposite side of the subsystem in a direction parallel to the firsttranslation axis, wherein a reference beam is provided which enters thesubsystem parallel to the beam of radiation from the source.
 9. Theapparatus of claim 8, further comprising a second optical subsystem formanipulating the source and reference beams between the source and thedetector, the first optical subsystem being contained within the secondoptical subsystem.
 10. The apparatus of claim 9, further comprisingmeans for translating the second subsystem along a second translationaxis relative to the fixed reference point, to scan the source beamacross the sample along the second translation axis, wherein source andreference beams each enter the second subsystem in a direction parallelto the second translation axis.
 11. The apparatus of claim 8, whereinthe reference radiation has a frequency of between 0.1×10¹² Hz and5×10¹⁴ Hz.
 12. The apparatus of claim 8, including a means forimplanting a predetermined delay into one of said source and referencebeams.
 13. An apparatus for investigating a sample, comprising: a sourceof a beam of radiation; a detector for detecting a beam of radiationreflected by or transmitted through the sample; an optical subsystem formanipulating the beam between the source and detector; and means fortranslating the optical subsystem along a first translation axisrelative to the sample to scan the beam across the sample; wherein thebeam from the source enters the subsystem on one side of the subsystemin a direction parallel to the first translation axis, and the beamreflected or transmitted exits the subsystem on the opposite side of thesubsystem in a direction parallel to the first translation axis, andfurther comprising a second optical subsystem, for manipulating thesource beam between the source and detector, and the first subsystembeing contained within the second subsystem.
 14. The apparatus of claim13, further comprising means for translating the second opticalsubsystem relative to the fixed reference point to scan the source beamacross the sample along a second translation axis, wherein the sourcebeam enters the second subsystem in a direction parallel to thedirection of translation of the second subsystem.
 15. The apparatus ofclaim 13, wherein the detector is within the second subsystem.
 16. Theapparatus of claim 13, wherein the detector is outside the secondsubsystem, and the reference beam and reflected or transmitted beam exitthe second subsystem parallel to the direction of translation of thesecond subsystem.
 17. The apparatus of claim 13, wherein the first andsecond directions of translation are orthogonal.
 18. An apparatus forinvestigating a sample, comprising: a source of a beam of radiation; asource of a reference beam; an optical subsystem for manipulating thesource beam; means for translating the optical subsystem along a firsttranslation axis relative to the sample to scan the beam across thesample; and a detector for detecting the reflected or transmitted beam;wherein source and reference beams each enter the subsystem in adirection parallel to the first translation axis.
 19. The apparatus ofclaim 18, wherein the detector is within the subsystem.
 20. Theapparatus of claim 18, wherein the detector is outside the subsystem,and the reference beam and reflected or transmitted beam exit thesubsystem parallel to the translation axis of the subsystem.
 21. Anapparatus for investigating a sample, comprising: a source of a beam ofradiation; a source of a reference beam; an optical subsystem formanipulating the beam between the source and a detector; and means fortranslating the subsystem relative to the sample; wherein reference beamenters the subsystem through an electromagnetic radiation guide, one endbeing fixed with respect the source of the reference beam, the other theguide being fixed relative to the subsystem, said reference beam beingconfigured to provide information concerning the phase of radiationleaving said source.
 22. The apparatus of claim 21, wherein the sourceis also outside the subsystem and the source beam enters the subsystemthrough an electromagnetic radiation guide.
 23. The apparatus of claim21, wherein each electromagnetic radiation guides is an optical fibre.24. An apparatus for investigating a sample, comprising: an emittercomprising a frequency conversion member for emitting a beam of THzradiation in response to irradiation by a pump beam of radiation; adetector configured to detect said beam of THz radiation using a probebeam of radiation; an optical subsystem comprising said emitter, saiddetector and means for manipulating the beam between the emitter, asample and the detector; the apparatus further comprising means fortranslating the subsystem relative to the sample such that the emitterand detector move together in a fixed relationship; a first opticalfibre with one end fixed with respect to a source of the pump beam, theother end fixed relative to the subsystem such that the pump beam entersthe subsystem through the first optical fibre; and a second opticalfibre with one end fixed with respect to a source of the probe beam andthe other end being fixed relative to the subsystem such that the probebeam enters the subsystem through the second optical fibre.
 25. Anapparatus according to claim 24, configured for imaging a sample.